Short answers, pulled from the story.
The earliest known iconographic representation of a chordophone is a 3,300-year-old stone carving of a Hittite bard playing a stringed instrument. This carving stands as the oldest known evidence of such an instrument, though the true lineage of the modern guitar remains shrouded in the mists of medieval Spain.
Literary sources credit the poet and musician Vicente Espinel with creating the five-course guitar, though his birth year of 1550 makes this claim impossible for some historians. The five-course baroque guitar was documented in Spain from the middle of the 16th century and enjoyed immense popularity in Spain, Italy, and France from the late 16th century to the mid-18th century.
Antonio Torres Jurado, a Spanish maker active between 1817 and 1892, increased the size of the guitar body, altered its proportions, and invented the breakthrough fan-braced pattern. This design greatly improved the volume, tone, and projection of the guitar and remains essentially unchanged since his time.
Steel guitar strings were introduced near the end of the 19th century in the United States, but nylon and steel strings became mainstream only following World War II. This shift marked a seismic change in the instrument's sonic capabilities and led to the development of the steel-string revolution.
By the 16th century, the guitar tuning of ADGBE had already been adopted in Western culture, and a lower E was later added on the bottom as a sixth string. This results in the standard tuning known today as EADGBE, which is a series of ascending fourths and a single major third from low to high.
George Beauchamp invented the first successful magnetic pickup for a guitar and incorporated it into the 1931 Ro-Pat-In, later known as Rickenbacker, Frying Pan lap steel. Electric guitars were first patented in 1937, using a pickup and amplifier that made the instrument loud enough to be heard in a band setting.